I've been pondering the fuel/heat trade off between S2 recovery from circular LEO and elliptical GTO. In some ways, LEO would be much like Dragon return from ISS so they have lots of data to work with on those conditions. GTO would be more like Apollo 13 Lunar return or Mars capture and I suspect there is much much less known about those condition. Obviously it can be done though. However, judicious timing of burns on GTO changes maximized the benefit for minimum fuel expended. Now consider doing it in reverse. Where is the sweet spot trading fuel to slow down with TPS to allow hotter entry? After all PICAX is supposed to be good for Mars return as I recall, and that's really trucking.

Also, if they only want to get one back for engineering study and not reflight (yet) is just needs to be in more-or-less one piece :-)

The Impetus is the very high launch rates of LEO sats. Make a dedicated stage for that, and just expend GTO stages. Seems to me like a better trade off.

Thats quite a good argument here. For the massive constellation, a dedicated second stage design would make sense. However, it doesnt solve the problem of near 0 payload when adding all the required bits to the current second stage. Dedicated design or not, there is just not enough margin. Maybe with a raptor, larger diameter, carbon fibre tank and a lot of luck. But that would take away development talent from ITS. Might be better to concentrate on an ITS version that can deploy stats. Should be easier than a design of a F9 reusable second stage.

@elonmuskConsidering trying to bring upper stage back on Falcon Heavy demo flight for full reusability. Odds of success low, but maybe worth a shot.

Any ideas how they might go about achieving this? Parachute landing, or propulsive landing? Deployable heat shield, or ablative-covered tanks? Probably going to need some serious heat protection as Stage 2 will need to re-enter at much faster than LEO speeds if it's sent a sat to GTO. Will need bigger batteries and maybe other consumables, too, to be able to still have power 4.5 hours or so after launch when it will be re-entering from a GTO mission...

@elonmuskConsidering trying to bring upper stage back on Falcon Heavy demo flight for full reusability. Odds of success low, but maybe worth a shot.

Any ideas how they might go about achieving this? Parachute landing, or propulsive landing? Deployable heat shield, or ablative-covered tanks? Probably going to need some serious heat protection as Stage 2 will need to re-enter at much faster than LEO speeds if it's sent a sat to GTO. Will need bigger batteries and maybe other consumables, too, to be able to still have power 4.5 hours or so after launch when it will be re-entering from a GTO mission...

For a first test, that's pretty easy, I'd do something that requires minimal modifications to the 2nd stage, like put a heat shield on the nose and attempt a controlled re-entry. If it survives reentry, you win, if it doesn't, you really haven't lost much of anything, and if the FH demo payload is a wheel of cheese or something similarly silly, then cutting into the payload margin doesn't matter much. The question really is - how long have they been preparing to attempt 2nd stage reuse? As with everything that SpaceX has done that surprises us, they may be much further along than any of us can guess.

Logged

"One bit of advice: it is important to view knowledge as sort of a semantic tree -- make sure you understand the fundamental principles, ie the trunk and big branches, before you get into the leaves/details or there is nothing for them to hang on to." - Elon Musk"There are lies, damned lies, and launch schedules." - Larry J

The Impetus is the very high launch rates of LEO sats. Make a dedicated stage for that, and just expend GTO stages. Seems to me like a better trade off.

Thats quite a good argument here. For the massive constellation, a dedicated second stage design would make sense. However, it doesnt solve the problem of near 0 payload when adding all the required bits to the current second stage. Dedicated design or not, there is just not enough margin. Maybe with a raptor, larger diameter, carbon fibre tank and a lot of luck. But that would take away development talent from ITS. Might be better to concentrate on an ITS version that can deploy stats. Should be easier than a design of a F9 reusable second stage.

Does it?

The mass penalty is 1:1, and current payload to LEO is 20 tons.

How much is a heat shield, a parachute? A ton?

Second stage, dry, is under 5 tons I believe. Suppose structural reinforcement is 20%. Another ton?

But without efficiently launching CommX there's no funding for Musk's ITS timeline, either.

What get's me is that the CommX deployment by SpaceX gives them good practice for doing the same thing around Mars.

I wouldn't be entirely suprised to see SpaceX putting multiple colonies at different locations to avoid any catastrophic situations that would wipe out all of the colonists.

People will die on Mars, that much is unavoidable. But putting everyone in one colony, with one TYPE of colony habitat, is way too much like putting all of your eggs in one basket. You're just asking for trouble.

Admittedly, spreading out the people to multiple colonies will cost more, but the chances to establish a foothold there increase dramatically by spreading them out and diversifying the approaches to how the colony is outfitted.

Yeah; nothing of what I just said has anything to do with second stage reusability, but if you think about it, the hypersonic reentry that the second stage would have to do, especially in the upper atmosphere, is not all together that different than the conditions that the ITS will have to go through to land on Mars.

Returning a second stage to Earth successfully would be good practice for landing the ITS itself.

"I think it would be great to be born on Earth and to die on Mars. Just hopefully not at the point of impact." -Elon Musk"We're a little bit like the dog who caught the bus" - Musk after CRS-8 S1 successfully landed on ASDS OCISLY

For a first test, that's pretty easy, I'd do something that requires minimal modifications to the 2nd stage, like put a heat shield on the nose and attempt a controlled re-entry.

This probably wouldn't work, though, without a lot of ballast. The engine is the heaviest part of the 2nd stage -- it needs to have the center of mass forward otherwise it would flip around on re-entry.

@elonmuskConsidering trying to bring upper stage back on Falcon Heavy demo flight for full reusability. Odds of success low, but maybe worth a shot.

Any ideas how they might go about achieving this? Parachute landing, or propulsive landing? Deployable heat shield, or ablative-covered tanks? Probably going to need some serious heat protection as Stage 2 will need to re-enter at much faster than LEO speeds if it's sent a sat to GTO. Will need bigger batteries and maybe other consumables, too, to be able to still have power 4.5 hours or so after launch when it will be re-entering from a GTO mission...

I don't think they would be able to do much modification for shielding... My assumption is that they would use mostly propulsive braking. A quick de-orbit burn, and then a 2nd longer burn as a pre-atmospheric braking burn (to kill virtually all velocity), and then hope that it stays intact until a parachute deploys.

@elonmuskConsidering trying to bring upper stage back on Falcon Heavy demo flight for full reusability. Odds of success low, but maybe worth a shot.

Any ideas how they might go about achieving this? Parachute landing, or propulsive landing? Deployable heat shield, or ablative-covered tanks? Probably going to need some serious heat protection as Stage 2 will need to re-enter at much faster than LEO speeds if it's sent a sat to GTO. Will need bigger batteries and maybe other consumables, too, to be able to still have power 4.5 hours or so after launch when it will be re-entering from a GTO mission...

I don't think they would be able to do much modification for shielding... My assumption is that they would use mostly propulsive braking. A quick de-orbit burn, and then a 2nd longer burn as a pre-atmospheric braking burn (to kill virtually all velocity), and then hope that it stays intact until a parachute deploys.

That's a huge amount of fuel of course. You're talking about the same delta-V, give or take, as S2 gave the payload on the way up (though with only 5 ton empty weight)

I don't know the exact numbers, but if it was 6 km/sec for the S2 contribution, and ISP =3000 sec, the mass ratio is 7:1...

So the starting mass in orbit is 35 tons...

So while maybe FH as a stunt can do it, it's not leading to a viable solution.

But adding 3 tons or so of re-entry hardware, or even 5, is still perfectly viable, since if the payload maximizes S2 performance, then you can choose to expend S2.

@elonmuskConsidering trying to bring upper stage back on Falcon Heavy demo flight for full reusability. Odds of success low, but maybe worth a shot.

Any ideas how they might go about achieving this? Parachute landing, or propulsive landing? Deployable heat shield, or ablative-covered tanks? Probably going to need some serious heat protection as Stage 2 will need to re-enter at much faster than LEO speeds if it's sent a sat to GTO. Will need bigger batteries and maybe other consumables, too, to be able to still have power 4.5 hours or so after launch when it will be re-entering from a GTO mission...

I don't think they would be able to do much modification for shielding... My assumption is that they would use mostly propulsive braking. A quick de-orbit burn, and then a 2nd longer burn as a pre-atmospheric braking burn (to kill virtually all velocity), and then hope that it stays intact until a parachute deploys.

The thing is that the rocket equation really hurts you when reserving propellant for upper stage return, while the first stage is more forgiving.

With an upper stage propulsive return, the more performance you need the more propellant you need to reserve for return. If your return relies primarily on adding a heat shield for aerobraking you don't need to reserve even more mass for high performance missions. That's why ITS is planning for propulsive landing on the first stage and using aerobraking on the upper stage.

I'm not totally clear how they would do it at all without fitting some kind of kit to give it legs and a second engine etc. (as we've discussed in many many threads) unless he means just seeing if they can splash it down gently (or land ON the bell which then gets squished)

Given that Musk has said it's as much about retiring ITS risk as F9 economics, how about a version of the roomba that's a prototype of the landing platform[1]... Obviates the need for legs.

And if the 1D can land the stage (see my post just above) that gets rid of the second engine leaving just TPS and, perhaps, grid fins as extra mass (plus the extra fuel).

We'll see in due course.

[1] To my eye those arms soooo look like they want to grab something hovering just above...